It seems that Coin Op Collection have completed a Snow Bros core for MiSTer before I did. Not really surprising since I have taken ages on this one. I had suffered from one of the same problems as they did, so at least I was on the right track.
For the moment I will look into the missing sound circuits on Cosmo (Space Invaders hardware) and then better analogue sound for Galaxian.
The 68000 based core I was working on is not totally redundant as it may be revisited later with a view to running a different game!
I have made a change to the checkout form for the shop, a phone number is now required. This is because shipping to some countries with Royal Mail now requires this and it saves me having to email you to ask for one.